1. Technical Field
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, and more particularly, to a copy machine, a printer, a facsimile machine, or a multi-functional system including at least two of these functions thereof.
2. Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multi-functional systems having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoreceptor (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the photoreceptor according to image data; a developing device supplies toner to the electrostatic latent image formed on the photoreceptor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoreceptor onto a recording medium or is indirectly transferred from the photoreceptor onto a recording medium via an intermediate transfer member using a transfer device; a cleaning device then cleans the surface of the photoreceptor after the toner image is transferred from the photoreceptor onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Conventionally, in such electrophotographic image forming apparatuses, constant current control of a direct current (DC) transfer bias that is applied to a transfer device using a DC power source is widely employed.
In recent years, however, a variety of recording media such as paper having a leather-like texture and Japanese paper known as “Washi” have come on the market. Such recording medium have a coarse surface due to fabrication such as embossing. However, toner does not transfer well to such recording medium, in particular recessed portions of the surface, causing gaps in an output image.
It is known that superimposing an alternating current (AC) voltage onto a DC voltage enhances toner transfer rate and prevents image defects. In such a configuration, changing transfer modes between a DC transfer mode and a DC-AC superimposed transfer mode (hereinafter referred to as superimposed transfer mode), in accordance with the type of recording medium leads to enhanced transferability of toner regardless of the type of recording medium.
In such a configuration, when transferring toner by superimposing the AC voltage onto the DC voltage, an AC component affects transfer of toner to the recessed portion of the recording medium and a DC component affects transfer of toner to the projecting portion. Therefore, there is a need to increase the strength of the DC bias to the transfer side according to the resistivity of the recording medium and the depth of the recessed portions in the surface of the recording medium (Refer to Voff in FIGS. 1(a) and (b)). At the same time, it is necessary that the polarity of the AC component opposite the polarity of the AC voltage at the toner transfer side that affects transfer of toner to the recessed portions remains constant (Refer to Vr in FIGS. 1(a) and (b)). Thus, in turn, there is a need to increase the peak-to-peak voltage (Refer to Vpp in FIGS. 1(a) and (b)) of the AC voltage. Accordingly, the polarity of the AC voltage at the toner transfer side (Refer to Vt in FIGS. 1(a) and (b)) increases. When a certain voltage is exceeded an electric discharge occurs, causing image degradation such as little or no settling of toner in the recessed portions and discharge images (so called white dot images) on the projecting portions.
Furthermore, it has been found that this electric discharge occurs because toner charge of the toner image before secondary transfer of the toner image is the same in both the DC transfer mode and the superimposed transfer mode, and for that reason the toner charge in the superimposed transfer mode is excessive.